1. Field of the Invention
This invention is in the field of apparatus used to mill a window through the wall of a casing in an oil or gas well, to allow for the drilling of a lateral bore hole.
2. Background Art
It is common in some oil or gas wells to drill a lateral bore hole extending out to the side of a vertical bore hole, usually for the purpose of accessing specific formations at different depths. This is often done by running a whipstock into the borehole and setting it at the desired depth. Setting of, the whipstock is followed by rotating a window mill in the borehole and following the curve of the whipstock through the casing wall, along a large radius path established by the whipstock. Once the window mill has penetrated the casing wall and created the window, the window mill is typically removed from the borehole and replaced by a drill bit which is then used to drill the lateral bore.
It is expensive to trip a work string in and out of a borehole for the purpose of installing the whipstock, running in the window mill, and running in the drill bit. It would be desirable to have a method and apparatus for accomplishing all of these operations in one trip.